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The drink gained its name from the effects of its bitter flavouring. The quinine was added to the drink as a prophylactic against malaria, since it was originally intended for consumption in tropical areas of South Asia and Africa, where the disease is endemic. Quinine powder was so bitter that British officials stationed in early 19th Century India and other tropical posts began mixing the powder with soda and sugar, and a basic tonic water was created. The first commercial tonic water was produced in 1858.[1] The mixed drinkgin and tonic also originated in British colonial India, when the British population would mix their medicinal quinine tonic with gin.

Since 2010, at least four tonic syrups have been released in the United States. Consumers add carbonated water to the syrup to make tonic water; this allows drinkers to vary the intensity of the flavour.[2]

Medicinal tonic water originally contained only carbonated water and a large amount of quinine. However, most tonic water today contains a less significant amount of quinine, and is thus used mostly for its flavor. As a consequence, it is less bitter, and is also usually sweetened, often with high fructose corn syrup or sugar. Some manufacturers also produce diet (or slimline) tonic water, which may contain artificial sweeteners such as aspartame. Traditional-style tonic water with little more than quinine and carbonated water is less common, but may be preferred by those who desire the bitter flavor.[citation needed]

In the United States, the US Food and Drug Administration (FDA) limits the quinine content in tonic water to 83 ppm[3] (83 mg per liter if calculated by mass), while the daily therapeutic dose of quinine is in the range of 500–1000 mg,[4] and 10 mg/kg every eight hours for effective malaria prevention (2100 mg daily for a 70 kg adult).[5] Still, it is often recommended as a relief for leg cramps, but medical research suggests some care is needed in monitoring doses.[6] Because of quinine's risks, the FDA cautions consumers against using "off-label" quinine drugs to treat leg cramps.[7]

Tonic water will fluoresce under ultraviolet light, owing to the presence of quinine. In fact, the sensitivity of quinine to ultraviolet light is such that it will appear visibly fluorescent in direct sunlight.

The quinine molecule absorbs light at approximately 350 nm and emits light at approximately 500 nm. For low concentrations of quinine, the intensity is directly proportional to the concentration. Over time, the transmission efficiency and the filters may be affected, meaning they may change. The transmission efficiency also changes with wavelength. The integration time affects signal/noise ratio, but not the signal magnitude. The detector is placed at 90° to the incident light beam to minimize the risk of transmitted or reflected incident light reaching the detector. The excited triplet state is lower in energy than the singlet state, which is why phosophorescence bands are found at longer wavelengths than the singlet state. The difference in wavelength can also be used to quantify the difference between the singlet and triplet state of the molecule.